Like all ticks, the American Dog Tick has life stages where its physical appearance slightly changes. The coloration darkens from nymph stage (see photos) to adulthood. One might mistake the two for different species.
Ticks take blood meals from the host animal they land on. They crawl to warm parts of the body (armpits and inner thighs on humans) and bite into the flesh using a numbing agent in the saliva that renders the puncture unnoticeable. They then proceed to suck the blood from the host until engorged and more than double its original size.
Ticks are notorious for carrying the bacterium that causes Lyme Disease in humans. Not all ticks are infected with them and not all bites of infected ticks result in Lyme Disease, but medical professionals recommend always seeing a doctor if you suspect, or confirm, a tick bite.